Protocols that achieve tolerance of kidney allografts in nonhuman primates (and in humans) fail to induce tolerance of heart allografts. We have taken advantage of the intrinsic tolerogenicity of kidney allografts and, through donor kidney cotransplantation, have achieved stable tolerance of heart allografts in nonhuman primates (NHP) which, if transplanted alone, are rejected acutely. The consistency with which kidney allografts confer tolerance upon recipients of cotransplanted heart allografts across different species (mouse, swine, NHP), across different histocompatibility barriers, and across different tolerance protocols is compelling. Registry studies furthermore confirm that human recipients of kidney and heart allografts from the same donor experience less acute rejection and less cardiac allograft vasculopathy (CAV) compared to those receiving heart transplants alone. Understanding the mechanisms underlying these unique observations has broad implications for the transplant community, well beyond the relatively small numbers of patients likely to receive kidney/heart cotransplants. Once these mechanisms are better understood, they could be exploited to guide novel therapeutic approaches that induce tolerance to heart allografts (and other tolerance-resistant organs) in the absence of a kidney, or improve outcomes without inducing tolerance. The mechanisms driving kidney- induced cardiac allograft tolerance (KICAT) and its implications for human heart transplant recipients is the focus of this Project. Our preliminary results in murine, swine, and NHP models implicate regulatory T cells (Treg) as the end effectors of KICAT with kidney-specific cells (e.g. plasmacytoid dendritic cells (pDC)) and/or cell products (i.e. erythropoietin (EPO)) amplifying those regulatory mechanisms. Indeed, emerging data from Heeger's group suggest that the unique ability of the kidney to induce tolerance is mediated in part by erythropoietin (EPO), a hormone traditionally thought to only be responsible for erythropoiesis but newly shown to function as a Treg-enhancing immunosuppressant. Together, our joint data support the following working model: high local concentrations of EPO in the donor kidney graft directly inhibit pathogenic effector T cells and promote the ability of kidney pDC to facilitate the generation and stability of donor-reactive Tregs via TGF? production. The enhanced activity of host Tregs induced by donor kidney elements leads to a robust state of heart allograft tolerance. Our goals are to test this model by determining the specific roles of Tregs, pDC, EPO and TGF? in achieving KICAT in NHPs and to characterize the robustness of the tolerance induced by KICAT in anticipation of clinical application. The newly acquired knowledge will be used to develop novel strategies to improve heart transplant outcomes and to induce tolerance of other highly resistant allografts.